This invention relates generally to surface mount technology in the electronic packaging area. More particularly, it relates to an improved method of aligning and mounting solder ball connectors to a surface of a substrate which will be subsequently joined through surface mount processes to another electronic structure.
Surface mount technology has gained acceptance as the preferred means of joining electronic devices together, particularly in high-end computers. As compared to more traditional pin connector methods, where a pin mounted to the backside of a ceramic module is thrust through a hole in the board, twice the number of modules can be placed at the same board area. Other advantages such as smaller component sizes, greater I/O densities, lower electrical resistance, decreased costs, and shorter signal paths have prompted the industry migration to surface mount technology.
A myriad of solder structures have been proposed for the surface mounting of one electronic structure to another. Typical surface mount processes form the solder structures by screening solder paste on conductive, generally metallic pads disposed on a surface of a first electronic structure, or "substrate". A stencil printing operation is used to align the contact mask to the pads. The solder paste areas on the substrate are aligned to and placed on corresponding pads on a second electronic structure, or "board". In some processes, solder paste may alternatively or additionally be screened on the board pads. After placement, the substrate and board go through a reflow operation to melt the solder paste and create a solder bond between the corresponding pads on substrate and board.
Other known surface mount technologies use solder balls rather than a solder paste to provide the solder structures. By using solder balls, a more exact and somewhat greater quantity of solder can be applied than through screening. The solder balls are aligned and are held to the substrate and melted to form the solder joint on the conductive pads. As before, the substrate with the newly joined solder balls is aligned to the board. The solder balls are then reflowed to form a good solder bond between substrate and board.
However, both the solder paste and solder ball surface mount techniques suffer when the density of the pads increase. A certain solder amount must be maintained to assure a reliable solder joint. As the required quantity of solder becomes large relative to the pad spacing, solder bridging between non corresponding conductive pads becomes a problem. The bridging problem is accentuated by the greater solder amount which is molten during the reflow process.
One method proposed in commonly assigned application Ser. No. 555,120, filed Jul. 18, 1990, entitled "Improved Interconnection Structure And Test Method", which is hereby incorporated by reference, uses a low melting point solder paste screened onto the substrate in conjunction with a high melting point solder ball. The use of a combination of a solder ball and solder paste allows a much greater amount of solder to be placed on the conductor pads due to the greater structural integrity of the solder ball during low temperature reflow. Using the solder ball solder paste combination, the solders can be tailored to provide the best electrical, structural and process characteristics. Referring to FIG. 1, the solder paste 15 applied to the conductive pads 16 of the substrate 17 is chosen to melt at a relatively low temperature, while the composition of the solder ball 11 melts at a high temperature, on reflow, only the small amount of the solder paste 15 melts, setting the solder ball 11 in place. Because only a small amount of the solder is reflowed, the threat of bridging is minimized, yet the reliability of having a large solder joint is achieved.
However, the manufacture of a solder joint using both solder paste and solder balls has proven difficult. Solder balls are difficult to align and handle during the reflow process. Different methods using vibration, brushing and vacuum in association with an alignment plate have been proposed for dealing with solder balls alone. The addition of the solder paste further complicates the process. Many problems were encountered maintaining the solder ball centrality with respect to each other and on the substrate, even to the extent that the solder balls were missing entirely. With the misalignment of the solder balls, bridging between adjacent pad sites became a problem. Good physical contact between the solder balls, solder paste and substrate must be assured while simultaneously preserving the alignment between substrate pads and solder joints. Process time mushroomed as the number of process checks increased.